We are taking this route in early September. We have a Eurail Selectpass. Should we make reservations for this itinerary?
Lauterbrunnen to Interlaken is just a milk-run commuter train. No reservations required or available. If you didn't have a railpass, you could get advance purchase tickets for Interlaken Ost to Munich for €39 pP.
When in Switzerland it's a good idea to have some sort of Swiss Pass product especially if you plan to take the high mountain private railways like the Jungfrau. Up to 5 persons can travel all the way from Munich to the Swiss Border on 1 Bayern Ticket for 29 Euro. Then use your Swiss Pass product like a Half Fare Card or Transfer Ticket from the Swiss Border all the way to Lauterbrunnen.